III-nitride semiconductors are very promising materials for use in high-electron mobility transistor (HEMT) structures, which typically include a carrier-supplying layer and a channel layer supporting the carrier-supplying layer. A semi-insulating buffer layer generally is required for electrical insulation of the HEMT structure from a substrate, and to provide tight charge control of the HEMT channel layer of the HEMT structure. Often, two types of gallium nitride (GaN) buffer layers are employed as the buffer layer: the first being a nominally-undoped GaN layer adjacent to the channel layer where intrinsic defects, such as dislocations and carbon impurity render semi-insulating properties; and the second being a GaN layer relatively remote from the channel layer that is intentionally doped with iron (Fe). The iron-doped buffer layer effectively isolates the HEMT structure from the underlying substrate by trapping free electrons in iron-impurity-related centers within the crystal structure.
The effectiveness of intentionally-doped buffer layers is often limited by the limited ability to control the concentration of doping during fabrication of the buffer layer. In particular, the profile of iron doping across an intentionally-doped buffer layer exhibits severe delay and a slow rise in the concentration of the iron upon introduction of iron during deposition of the layer and also very slow decay in the concentration of iron after terminating introduction of iron to the vapor from which the buffer layer is deposited. For example, as demonstrated by Rudzinski et al., Phys. Stat. Sol. (c) 3, 6, 2231-2236 (2006), after the introduction of iron to a gaseous phase above a buffer layer has terminated, the concentration of iron over a large thickness of GaN (e.g. 1 μm or larger) decreases very slowly. This slow diminishment in iron concentration has been attributed to segregation of iron on the surface of GaN as it forms (Heikman et al., App. Phys. Lett. 81, 439 (2002)). Moreover, if decay in iron concentration of a nominally-undoped buffer layer is not complete, iron can become incorporated into the overlying channel layer, thereby degrading performance characteristics of the resulting HEMT structure, as demonstrated by Desmaris et al. IEEE Trans. Electron Devices 52, 9, 2413-2417, (2006).
One attempt to solve the problem of iron contamination of the HEMT channel layer is to concentrate iron at a portion of the buffer layer that is relatively close to the substrate, and to leave the portion of the buffer layer relatively close to the channel layer essentially undoped. As a consequence, the GaN buffer layer has a concentration of iron doping that is modulated across the thickness of the buffer layer, whereby the highest concentrations of iron doping are relatively remote from the overlying channel layer. One limitation that is commonly associated with such modulated iron-doping is that the resulting HEMT structure is vulnerable to excessive leakage and poor device pinch-off due to undesirable electron conduction in the relatively undoped part of the GaN buffer layer.
Therefore, a need exists for a gallium nitride-based HEMT structure within an iron-doped buffer layer, and for methods of forming such structures, that overcome or minimize the above-referenced problems.